Ongoing studies of the organization and expression of endogenous xenotropic MuLV DNA sequences have highlighted the need for cloning each of the 28 defined HindIII fragments found in BALB/c mice. The availability of each of these fragments would allow us to better define the role of each proviral MuLV sequence along with its flanking DNA in virus expression. We have used the cosmid system to begin our studies and preliminary data suggests we have cloned the most prominent fragment (ll.6 KB) which is not only found in BALB/c mice but is common to most mice tested.